Drillrods for use in oil and natural gas exploration are manufactured according to the API SPEC 5DP standards. The structure thereof has an externally threaded drillrod coupler and an internally threaded drillrod coupler which are respectively frictionally butt-welded at the two ends of the drillrod tube body. Drillrods in compliance with the API SPEC 5DP standards are of a low alloy steel material.
With the development of the oil industry, the conditions in which drillrods operate become more and more severe, drillrods of the low alloy steel material as per the API SPEC 5DP standards now fail to fulfill the increasingly harsh requirements of well drilling operation, and there exists an urgent need for a high alloy drillrod. To this end, aluminum alloy drillrods and titanium alloy drillrods appeared on the market. The aluminum alloy drillrods are manufactured as per the ISO 15546 standards. The aluminum alloy drillrod is formed from an aluminum alloy drillrod tube body connected by means of fine threads with an externally threaded coupler made of low alloy steel and an internally threaded coupler made of low alloy steel. The structure of the titanium alloy drillrod is similar to that of the aluminum alloy drillrod.
The utilization of the aluminum alloy drillrod and the titanium alloy drillrod has two major objectives as follows: one is to drill a super deep well by taking advantage of the low specific gravity property of the aluminum alloy drillrod and the titanium alloy drillrod, and the other is to drill a sulfur-containing well by taking advantage of the resistance property of the aluminum alloy drillrod and the titanium alloy drillrod to stress corrosion by sulfides.
For some CO2-containing gas fields whose stratum is of compact sandstone, in the case of a conventional method of operation which employs a drillrod for drilling a well and an oil tube for completing the well, the yield is only tens of thousands of cubic meters/day; in addition, superior 13Cr high alloy oil tube products must be used in a gas field containing a relatively high level of CO2, resulting in an extremely low yield of production and an extremely high cost, meaning low value in industrial exploration.
If a nitrogen well-drilling process can be employed, the above-mentioned problem can be solved and a high yield of millions of cubic meters of natural gas per day can be achieved. However, when the nitrogen well-drilling process is used, the drillrod cannot be lifted out to exchange into the oil tube for well completion, otherwise the production layer would be contaminated, lowering the yield back to tens of thousands of cubic meters/day. This gives rise to the need of a superior 13Cr high alloy drillrod product resistant to CO2 corrosion. The product is required to not only be able to be used as a drillrod in an earlier stage of nitrogen well-drilling operation, and also be able to be used as an oil tube in a later stage of well completion with oil tube.